Two scientists placed human subjects and snakes inside a MRI machine to measure how the human body responds to fear. Scientific American reports how they did it, and why:

You are in an MRI machine. Your head is fixed in a round cage. Your body is rolled into a narrow tube. Magnetic pulses are beamed into your brain. A meter-and-a-half-long snake is strapped with Velcro atop a small box on a conveyor belt just inches behind your head. Your eyes meet the snake’s beady gaze through a tiny mirror above your head. You can’t move.

Why would Uri Nili and Yadin Dudai, two scientists from the Weizmann Institute of Science in Rehovot, Israel, want to put a snake in the MRI scanner with you? Obviously, not to scan the snake’s brain (although this might be an interesting possibility). They wanted to scan your brain while you perform an act of courage. They wanted to push research on fear one step further – from understanding how we passively react to fear, through actively avoiding it, to actually confronting it.

The subject could choose to move the snake closer or farther away in increments that the researchers called "snake-advance units". As the subject altered the relative position of the snake, instruments examined changes in his/her body.